This invention relates to boiler control systems for boilers fired by solid fuels where the solid fuels are carried through the combustion zone by a traveling grate and wherein the air flow for combustion is supplied to the undergrate region. It has been well known in the art to control the air flow in the undergrate region of such boilers in direct proportion to the steam flow from the boiler so as to maintain the desired air to fuel ratio as the load on the boiler varies.
This method for controlling the flow of undergrate air, however, has not proved to be adequate for the maintenance of close control of steam pressure in systems where the supply of solid fuel to the grate is controlled in response to deviations of the steam outlet pressure. This results from the inherent lag in the change in heat released to the boiler when a change has been made in the fuel feed rate in response to the deviation from the desired steam outlet pressure. These lags, of course, result from the fact that the traveling grate requires a certain amount of time to provide a change in BTU input to the boiler.
It is thus an object of the present invention to provide an improved control system for a boiler firing solid fuels onto a traveling grate to provide closer control of the steam outlet pressure where the rate of fuel feed is being controlled from outlet pressure deviation.